WD-script-960808
WD-script-960808This is a W3C Working Draft for review by W3C HTML-ERB members. The only difference from the previous draft is that we have added some Tcl examples generously provided by Jacob Levy of Sun Microsystems.
The HyperText Markup Language (HTML) is a simple markup language used to create hypertext documents that are portable from one platform to another. HTML documents are SGML documents with generic semantics that are appropriate for representing information from a wide range of applications. This specification extends HTML to support locally executable scripts including JavaScript, VBScript, and other scripting languages and systems.
This specification extends HTML to support client-side scripting of HTML documents and objects embedded within HTML documents. Scripts can be supplied in separate files or embedded directly within HTML documents in a manner independent of the scripting language. Scripts allow HTML forms to process input as it is entered: to ensure that values conform to specified patterns, to check consistency between fields and to compute derived fields.
Scripts can also be used to simplify authoring of active documents. The behaviour of objects inserted into HTML documents can be tailored with scripts that respond to events generated by such objects. This enables authors to create compelling and powerful web content. This specification covers extensions to HTML needed for client-side scripting, but leaves out the architectural and application programming interface issues for how scripting engines are implemented and how they communicate with the document and other objects on the same page.
<!-- SCRIPT is a character-like element for embedding script code
that can be placed anywhere in the document HEAD or BODY -->
<!ELEMENT script - - CDATA>
<!ATTLIST script
type CDATA #IMPLIED -- media type for script language --
language CDATA #IMPLIED -- predefined script language name --
src %URL #IMPLIED -- URL for an external script --
>
The content model for the SCRIPT element is defined as CDATA. In this kind of element, only one delimiter is recognized by a conforming parser: the end tag open (ETAGO) delimiter (i.e. the string "</"). The recognition of this delimiter is constrained to occur only when immediately followed by an SGML name start character ([a-zA-Z]). All characters which occur between the SCRIPT start tag and the first occurrence of ETAGO in such a context must be provided to the appropriate script engine.
Note that all other SGML markup (such as comments, marked sections, etc.) appearing inside a SCRIPT element are construed to be actual character content of the SCRIPT element and not parsed as markup. A particular script engine may choose to treat such markup as it wishes; however a script engine should document such treatment.
The restriction on appearance of the ETAGO delimiter may cause problems with script code which wishes to construct HTML content in the code. For example, the following code is invalid due the to presence of the "</EM>" characters found inside of the SCRIPT element:
<SCRIPT type="text/javascript">
document.write ("<EM>This won't work</EM>")
</SCRIPT>
A conforming parser would treat the "</EM>" data as an end tag and complain that it was an end tag for an element not opened, or perhaps actually close an open element. In any case, it is recognized as markup and not as data.
In JavaScript, this code can be expressed legally as follows by ensuring that the apparent ETAGO delimiter does not appear immediately before an SGML name start character:
<SCRIPT type="text/javascript">
document.write ("<EM>This will work<\/EM>")
</SCRIPT>
In Tcl this looks like:
<SCRIPT type="text/tcl">
document write "<EM>This will work<\/EM>"
</SCRIPT>
In VBScript, you can avoid the problem with the Chr() function, e.g.
"<EM>This will work<\" & Chr(47) + "EM>"
Each scripting language should recommend language specific support for resolving this issue.
The following describe the attributes used with SCRIPT elements, all of which are optional:
Using text as the primary media type has the advantage of allowing people to view scripts as text files. An alternative would be to register "script/tcl" or "application/python" etc.
HTML documents can include multiple SCRIPT elements, which can be placed in the document HEAD or BODY. This allows script statements for a form to be placed near to the corresponding FORM element. Note that some script engines evaluate script statements dynamically as the document is loaded, so that there is the possibility that references to objects occurring later in the document will fail.
The default scripting language in the absence of TYPE or LANGUAGE attributes can be specified by by a META element in the document HEAD, for example:
<META HTTP-EQUIV="Content-Script-Type" CONTENT="text/tcl">
where the CONTENT attribute specifies the media type for the scripting language, and the HTTP-EQUIV attribute is the literal string "Content-Script-Type". If there are several such META elements the last one determines the scripting language.
In the absence of such a META element, the default can be set by a Content-Script-Type HTTP header in the server response, for example:
Content-Script-Type: text/tcl
If there are several such headers, the last one takes precedence over earlier ones. In the absence of either a META element or an HTTP header, the default scripting language type is assumed to be JavaScript.
Some scripting languages permit script statements to be used to modify the document as it is being parsed. For instance, the HTML document:
<title>Test Document</title>
<script type="text/javascript">
document.write("<p><b>Hello World!<\/b>")
</script>
Has the same effect as the document:
<title>Test Document</title>
<p><b>Hello World!</b>
From the perspective of SGML, each script element is evaluated by the application and can be modelled as a two-step process: (1) dynamically defining an anonymous CDATA entity; corresponding to the combined text written by all of the document.write or equivalent statements in the script element and (2) referencing the entity immediately after parsing the script end tag. HTML documents are constrained to conform to the HTML document type definition both before processing any script elements and after processing all script elements.
<!ELEMENT noscript - - (%body.content)>
The content of this element is rendered only when the user agent doesn't support client-side scripting, or doesn't support the scripting langauge used by preceding script elements in the current document. It gives authors a means to provide an invitation to upgrade to a newer browser, and is designed to work with downlevel browsers. The NOSCRIPT element can be placed anywhere you can place an HTML DIV element.
<NOSCRIPT>
<P>This document works best with script enabled browsers
</NOSCRIPT>
You can include the handler for an event in an HTML document using the SCRIPT element. Here is a VBScript example of an event handler for a text field:
<INPUT NAME=edit1 size=50>
<SCRIPT TYPE="text/vbscript">
Sub edit1_changed()
If edit1.value = "abc" Then
button1.enabled = True
Else
button1.enabled = False
End If
End Sub
</SCRIPT>
Here is the same example using Tcl:
<INPUT NAME=edit1 size=50>
<SCRIPT TYPE="text/tcl">
proc edit1_changed {} {
if {[edit value] == abc} {
button1 enable 1
} else {
button1 enable 0
}
}
</SCRIPT>
Here is a javascript example for event binding within a script: First, here's a simple click handler:
<script language=JavaScript>
function my_onclick() {
. . .
}
document.form.button.onclick = my_onclick
</script>
Here's a more interesting window handler:
<script language=JavaScript>
function my_onload() {
. . .
}
var win = window.open("some/other/URL")
if (win) win.onload = my_onload
</script>
In Tcl this looks like:
<script language=tcl>
proc my_onload {} {
. . .
}
set win [window open "some/other/URL"]
if {$win != ""} {
win onload my_onload
}
</script>
Scripting engines are responsible for binding object references in scripts to objects associated with documents. Script engines may support more than one language. This allows handlers to be written in one language, and the event binding to be defined in another, thereby avoiding the limitations of particular languages.
Some scripting languages like VBScript provide language conventions for binding objects that source events to script functions that handle events. Other languages typically allow you a run-time mechanism to set up such bindings, e.g. to register call-backs, or a way to poll for events and dispatch them to appropriate handlers.
How do scripts reference objects? In many cases objects associated with HTML elements such as form fields can be identified by virtue of the document markup, e.g. the tag names and attribute values. HTML ID attributes provide identifiers that are unique throughout a given document, while NAME attributes for elements defining form fields are limited in scope to the enclosing FORM element.
Scripting systems may allow authors to script objects that occur within an object associated with an OBJECT or IMG element. Document frames allow one document to be nested in another. Script handlers could be placed in a parent document and used to control the behaviour of a child document. Scripts may also be used for objects external to documents, such as the user agent or other applications. A particularly simple form of scripting is to just wire up objects that source events with ones that sink events. One event may be multicast to several recipient objects.
One way to deal with naming is to introduce language specific naming conventions, e.g. "document.form1.button1" as used by JavaScript. Another is to rely on the context in which a SCRIPT element is located to guide search for a named object. For instance, if the SCRIPT element is within a FORM element, objects associated with form elements with matching NAME values may be sought in preference to elements with matching ID values.
Some scripting languages, such as VBScript, limit the scope of references to a given module, but don't provide language specific means for defining modules. If a module is associated with an HTML document then element ID values can be used for binding handlers to objects. If the module is associated with an HTML form, then form field NAME attribute values could in principle be used by the script engine to unambigously bind handlers to objects by placing the handlers in a SCRIPT element within the associated FORM element.
A number of common events can be handled using attributes placed on the HTML elements associated with the object generating the event. The attribute names for intrinsic events are case insensitive. The attribute value is a scripting language dependent string. It gives one or more scripting instructions to be executed whenever the corresponding event occurs. Note that document.write or equivalent statements in intrinsic event handlers create and write to a new document rather than modifying the current one.
In the following example, userName is a required text field. When a user attempts to leave the field, the OnBlur event calls a JavaScript function to confirm that userName has an acceptable value.
<INPUT NAME="userName" onBlur="validUserName(this.value)">
Here is another JavaScript example:
<INPUT NAME="num"
onChange="if (!checkNum(this.value, 1, 10))
{this.focus();this.select();} else {thanks()}"
VALUE="0">
The scripting language assumed for intrinsic events is determined by the default scripting language as specified above for the SCRIPT element.
The script attributes for intrinsic events are defined as CDATA. The SGML processing of CDATA attribute values requires that (1) entity replacement occur within the attribute value; and (2) that the attribute value be delimited by the appearence LIT ( " ) or LITA ( ' ). The literal delimiter which terminates the attribute value must be the same as the delimited used to initiate the attribute value. Given these lexical restrictions, the delimiters LIT or LITA, ERO (entity reference open - '&'), and CRO (character reference open - "&#") may not freely occur as script code within a script event handler attribute. To resolve this issue, it is recommended that script event handler attributes always use LIT delimiters and that occurrences of '"' and '&' inside an event handler attribute be written as follows:
'"' should be written as """ or as """
'&' should be written as "&" or as "&"
For example:
<INPUT NAME="num"
onChange="if (compare(this.value, "help")) {gethelp()}"
VALUE="0">
Note that SGML permits LITA ( ' ) to be included in attribute strings quoted by LIT ( " ), and vice versa. The following is therefore okay:
"this is 'fine'" and 'so is "this"'
The following is an example of how intrinsic events are specified in the HTML document type definition:
<!ATTLIST SELECT
name CDATA #REQUIRED
size NUMBER #IMPLIED
multiple (multiple) #IMPLIED
onFocus CDATA #IMPLIED
onBlur CDATA #IMPLIED
onChange CDATA #IMPLIED
>
The set of intrinsic events are listed below together with the HTML elements they can be used with. This set is expected to grow slightly:
Note it would be very useful to be able to respond to changes to input fields on a key stroke by key stroke basis. This would allow authors to constrain input dynamically to fit mask patterns etc. This need could be met by altering the semantics of onChange or adding a new intrinsic event.
This specification allows macros to be included as part of any HTML attributes defined as CDATA in the document type definition. This allows attributes to be set depending on the properties of objects that appear earlier on the page. The syntax is:
attribute = "... &{ macro body } ... "
The macro body is made up of one or more statements in the default scripting language (as per instrinsic event attributes). The processing of CDATA attributes proceeds as follows:
Note that macro processing takes place when the document is loaded (or reloaded) but isn't redone when the document is resized or repainted etc.
examples needed here ...
For an INPUT, TEXTAREA or SELECT element to be considered as part of a form when submitting the form's contents both of the following conditions must apply:
If either of these two conditions are not met, then the field is not treated as being part of a form. This allows fields such as text fields and buttons to be used together with scripting to build user interfaces independent of the role of these elements for forms.
Authors may wish to design their HTML documents to be viewable on older browsers that don't recognise the SCRIPT element. Unfortunately any script statements placed within a SCRIPT element will be visible to users. Some scripting engines for languages such as JavaScript and VBScript allow the script statements to be enclosed in an SGML comment syntax, for instance:
<SCRIPT LANGUAGE="JavaScript">
<!-- to hide script contents from old browsers
function square(i) {
document.write("The call passed ", i ," to the function.","<BR>")
return i * i
}
document.write("The function returned ",square(5),".")
// end hiding contents from old browsers -->
</SCRIPT>
The JavaScript engine allows the string "<!--" to occur at the start of a SCRIPT element, and ignores further characters until the end of the line. JavaScript interprets "//" as starting a comment extending to the end of the current line. This is needed to hide the string "-->" from the JavaScript parser.
Down-level browsers will ignore the SCRIPT start and end tags and interpret the "<!--" string as the start of an SGML comment. In this way the contents of the SCRIPT element are hidden within an SGML comment.
The same example in Tcl looks like:
<SCRIPT LANGUAGE="tcl">
<!-- to hide script contents from old browsers
proc square {i} {
document write "The call passed $i to the function.<BR>"
return [expr $i * $i]
}
document write "The function returned [square 5]."
// end hiding contents from old browsers -->
</SCRIPT>
In VBScript a single quote character causes the rest of the current line to be treated as a comment. It can therefore be used to hide the string "-->" from VBScript, for instance:
<SCRIPT TYPE="text/vbscript">
<!--
Sub foo()
...
End Sub
' -->
</SCRIPT>
Some browsers close comments on the first ">" character, so to hide script content from such browsers, you can transpose operands for relational and shift operators (e.g. to use "y < x" rather than "x > y") or to use scripting language dependent escapes for ">".
The World Wide Web Consortium:
http://www.w3.org/